I Fly...
That is not the reason for the OM/DME altimeter check.
The combination of localizer (azimuth), glideslope (elevation) and OM/DME (distance) intersect each other at virtually 90 degrees and when you are at that X,Y,Z intersection you are in a very small box, a much smaller 'circular area probable' box than even a Differential GPS can put you in. Certainly smaller than IFR altimeter tolerances.
There are two reasons for the check.
1. For you to ensure that you are on the correct lobe for glideslope guidance. Your NAV equipment can possibly capture a 'false' lobe above or below the 3 degree path that you want to be on. This is also the reason that you were (probably) taught to intercept the glideslope 'from below.'
An example of a false lobe is if you have ever had a false localizer capture, where you are say on an arc to intercept the localizer, you haven't passed the lead bearing yet the localizer bar comes alive. Something I've seen about every one in three times intercepting the Cairns 33 LLZ/DME from the southwestern arc.
2. To ensure that in "ISA minus lots" conditions, the sort of cold conditions that we don't see much of in Oz, that you do not descend below the Cat I Decision
Altitude.
Presuming you are on-slope at the OM, if your altimeter indicates a
higher altitude than the OM check altitude, you should add the difference to your DA, to prevent you going below the Cat 1 minima.
The airlines do not use Radar Altimeter height in Oz, that is a Cat 2 ILS procedure.
The then-CAA used to publish pilot notes on the ILS, VOR and NDB. Worth a read.
Your comments about altimeters as 'rubber bands' are true, but a little bizarre in the context of this forum. Not too many readers here are cruising above 100,000'. A standard altimeter is considered good enough for instrument approaches close to the ground where Airservices charts round figures to 5', it is good enough for VFR/IFR separation of 500' up to FL200, and only starts to run out of puff if you want to use RVSM airspace at or above FL290 when you start being separated from other IFR by 2000'.
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I disagree with the other comments such as "the rules are confusing and contradictory, therefore I will apply my own standard."
Granted, we don't/didn't get paid super salaries in GA, but the courts and the regulator don't give two hoots about that.
The rules are in the books, and the books are out there to be read. Not as easy to read as a Picture magazine, but there they are and the presumption is that they be read by us.
It is also presumed that our personal standards are built on a good knowledge of the rules, not as a reaction to them.
Now, let me give you an example and see what you guys think.... what would you REALLY do here?
Your flight started out as a nice little jaunt. Low cloud at your destination, but stable conditions indicated in the forecasts on your briefing. Your knowledge of the route and the destination gives you no cause to doubt that this will be a routine journey.
You have a maximum pax load, and therefore are carrying just the fuel required by your calculations plus a small margin, you could even nick off to a nearby military base if something prevents you from landing at your planned destination at the last minute. In other words, you have fuel to meet the requirements as per your valid brief, plus a little bit of margin.
OK, as you pass your last point of safe diversion to an off track field, you get a call from ATC and they inform you that conditions at your destination have deteriorated significantly. The conditions are currently fluctuation at or about the minima, and are expected to worsen and eventually sock in. The nearby military field is already at minima and will also shortly be unavailable.
After saying "Fire truck" a few times and double checking ATIS's, AERIS and FlightWatch for all possibles, you realise you don't have enough fuel to divert anywhere in the clear.
You do see though that if you proceed, you will have one good shot at the runway before it really craps in.
You fly the ILS, and man its a rough old night. Gusting winds, the whole lot.
About halfway down, due to a bit of terrain or whatever, the crosswind that you were holding off a bucketload of drift for, disappears. You shoot out the 'windward' side of the localizer, you pass half scale deflection.
Now, according to the 'personal standards' advocates, they would go around.
But if they knew from the books what they were
allowed to do, such as exceed a half scale deflection so long as the error is not sustained and does not exceed full scale deflection, they would know in the heat of that moment that they could make a heading change, get back on the localizer and continue the approach to a reasonable chance of getting visual at or before the DA instead of going around and rooting their last chance, WHEN THERE WAS NO REQUIREMENT TO DO SO.
